Veertu (for Mac)

Pros

Fast and reliable.
Sandboxed security.
Free version includes easy downloads of Linux and other free OS guests.
Simple interface.

Cons

Minimal integration between host OS X and guest Windows or Linux system.
No drag-and-dropping between host and guest.
Only one shared folder for Windows guests.

Bottom Line

Veertu is a minimalist virtualization app for running Windows, Linux, and a few other operating systems under OS X.
It lacks some features offered by the competition, but it's got excellent sandboxed security built in.